This section provides background information related to the present disclosure which is not necessarily prior art.
Hydrogen is an attractive fuel as it can provide low emissions and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device having an anode and a cathode separated by an electrolyte. The anode receives a fuel such as hydrogen gas and the cathode receives an oxidant such as oxygen or air. Hydrogen gas is dissociated in the anode to generate free protons and electrons, where the protons pass through the electrolyte to the cathode. The electrons from the anode do not pass through the electrolyte, but are instead directed through a load to perform work before being directed to the cathode. In the cathode, the protons, electrons, and oxygen react and generate water.
Proton exchange membrane (PEM) fuel cells are a type of fuel cell used to power vehicles. The PEM fuel cell generally includes a solid polymer-electrolyte proton-conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode can include a catalytic mixture of finely divided catalytic particles, such as platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture can be deposited on opposing sides of the membrane. Combination of the anode catalytic mixture, the cathode catalytic mixture, and the membrane can be referred to as a membrane electrode assembly (MEA).
Several fuel cells can be combined into one or more fuel cell stacks to generate the desired power. For certain applications, a fuel cell stack may include several hundred or more fuel cells. The fuel cell stack receives a cathode reactant gas, which can be a flow of air forced through the stack by a compressor. The fuel cell stack also receives an anode reactant gas such as hydrogen that flows into the anode side of the stack.
A fuel cell stack can include a series of bipolar plates positioned between several MEAs within the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates to allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates to allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates can also include coolant flow channels, through which a cooling fluid flows to control the temperature of the fuel cell.
Reactant gas stream passages can be sealed to prevent leaks and intermixing of fuel and oxidant fluid streams. Fuel cell stacks can employ resilient seals or gaskets between stack components to provide effective sealing. Such seals can also isolate manifolds and electrochemically active areas of the fuel cell MEAs by circumscribing these areas. For example, a fluid tight seal can be achieved by a resilient gasket formed of an elastomeric material interposed between the flow field plates and the membrane to which a compressive force is applied. A fuel cell stack having multiple plates can therefore be equipped with multiple gaskets and a suitable compression system for applying a compressive force to the gaskets.
Sealing around plate manifold openings and MEAs within fuel cells can include framing the MEA with a fluid-impermeable and resilient gasket, placing preformed gaskets in channels in the electrode layers and/or separator plates, or molding seals within grooves in the electrode layer or separator plate, and circumscribing the electrochemically active area and any fluid manifold openings.
Seals can also be formed using cure-in-place and form-in-place methods. Cure-in-place methods dispense a bead of precursor material, such as silicone elastomer, to provide a seal between two or more components. The cure-in-place process can be used where parts may be disassembled for service. A bead of precursor is dispensed in specifically defined areas of one of the components to be assembled. Once the precursor has cured and is adhered to the component to which it was applied, the component is assembled with the other component(s) and the cured bead is compressed. The compression and the cohesion of the cured material provides the sealing. However, with cure-in-place sealing it can be difficult to reliably meet tight gasket height tolerances (e.g., 1.050±0.10 mm) for the dispensed bead of precursor. Slow and costly robotic dispensing speeds (e.g., about 60 sec cycle time per plate) are hence used to obtain the desired tolerance.
Form-in-place methods dispense a bead of precursor material, such as a silicone elastomer, during the assembly of two or more stack components. The precursor is dispensed on the surface of one or more components and the components are then assembled while the precursor is still wet and uncured. Once the precursor has cured, elastomeric cohesion provides sealing between the components. This technique can be used where components are assembled immediately on a production line and for parts that do not need to be frequently disassembled. Use of the form-in-place sealing process can relax the dispensed gasket height tolerance about 10-fold (e.g., 2.50±1.0 mm), which in turn reduces the sealing cycle time by half (e.g., about 30 sec cycle time per plate). However, with form-in-place sealing, the metal bipolar plates in a fuel cell may not be stiff and flat enough, making it difficult to control the uncured and wet gasket thickness upon stack assembly.